Blender 2.8 (python) - how do I get the vector of the direction the camera points
Since a clear rotation camera (0,0,0) point down z-axis.

So the vector should be $camera\ rotation\ matrix\times Vector(0,0,-1)$

In code:

v = Vector((0,0,-1)) #Down vector
v.rotate(camera.rotation_euler)

v #this vector



  Except
  
  Keep in mind that camera can have another transform by parent offset and some other modifier. If the camera rotation is not pure, you will need to apply the transform to that rotation as well.